Re: Macros considered harmful

[Philip Kaludercic <philipk@posteo.net>]

"Basil L. Contovounesios" <contovob@tcd.ie> writes:

> Stefan Monnier [2022-09-06 00:44 -0400] wrote:
>
>> Clément Pit-Claudel [2022-09-05 19:53:26] wrote:
>>> On 8/20/22 07:14, Stefan Monnier wrote:
>>>> FWIW, I think specifying the highlighting rules with something akin to:
>>>>      (defvar <foo> '<rules>)
>>>> is a mistake.  It should go through some kind of macro, such as (maybe):
>>>>      (defvar <foo> (tree-sitter-rules <rules>))
>>>> which can thus do any preprocessing we may want, such as pre-compiling
>>>> queries.  It also helps evolve the syntax since we can more easily warn
>>>> about obsolete uses, etc...
>>>> I've had a "rewrite font-lock.el so the rules go through a macro" in my
>>>> todo list for ages.
>>>
>>> We do things this way in Flycheck, but we've been bitten a few times by the
>>> way this pattern interacts with `with-eval-after-load`, so I would be
>>> careful about adopting this pattern in tree-sitter (unless we expect it to
>>> be preloaded).
>>
>> Very good point.  It would introduce a strong compile-time dependency on
>> the tree-sitter package, which could be a serious annoyance for random
>> `foo-mode` packages which want to keep working in the absence of
>> tree-sitter.
>>
>> Hmm...
>>
>> [ I guess we'd have to make that macro lightweight and independent from
>>   tree-sitter itself and put it into a separate package distributed via
>>   GNU ELPA, so packages can feel free to depend on it.
>>   Hmm...  Another problem.  ]
>>
>>> In fact, I think your suggestion back then was to *not* use a macro?
>>
>> Indeed,
>>
>> [ Lua eshews macros altogether for those kinds of reasons, AFAIU.  ]
>>
>> Admittedly, another way around these kinds of problems is to teach the
>> compiler how to deal with an unknown macro.  I.e. something like
>> (declare-macro my-foo ...) so that if the compiler see (my-foo ...) but
>> `my-foo` can't be macroexpanded (because the macro is not yet defined),
>> it doesn't incorrectly compile it into a function call, but instead
>> residualizes it into something like a call to `eval`.  Making it
>> interact correctly with lexical scoping could be tricky (I guess the
>> simplest solution would be to residualize the whole toplevel expression
>> in which the macro call was found).
>
> Another downside of macros not directly addressed by this approach is
> that packages using them may have the outrageous desire to both support
> older Emacsen and build cleanly, at the same time!  Recall, for example,
> this unresolved shortdoc thread:
> https://lists.gnu.org/r/emacs-devel/2021-09/msg01719.html
>
> I suppose in the general case this can be alleviated only through
> 'compat'-like ELPA dependencies (or by expecting each package to write
> its own compatibility shims, of course).

As the maintainer of the package "compat", I'd be more than happy to
help out with these issues.